The basic estimate
Battery life is capacity divided by average draw:
hours ≈ capacity (mAh) × usable fraction ÷ average current (mA)
A 2500 mAh AA driving a steady 20 mA, derated to 80%, lasts 2500 × 0.8 / 20 = 100 hours. The usable fraction matters: no battery delivers its full nameplate capacity down to 0 V, so a derating factor keeps the estimate honest.
Intermittent loads (the duty-cycle mode)
Most battery-powered gadgets aren't on continuously — a sensor node might wake for 2 s to transmit at 80 mA, then sleep at 10 µA for five minutes. The average current is the time-weighted blend:
I_avg = (I_active × t_active + I_sleep × t_sleep) / period
For that example: (80 mA × 2 s + 0.01 mA × 298 s) / 300 s ≈ 0.54 mA average — so the sleep current, tiny as it is, still matters once the active burst is brief. This is why low sleep current dominates the battery life of well-designed IoT devices.
Why you won't get the nameplate capacity
- Cutoff voltage — your circuit stops working before the cell is fully flat, stranding some capacity.
- Temperature — cold cuts usable capacity substantially, especially for alkaline and Li-ion.
- Self-discharge — Li-ion loses ~2–3%/month, alkaline slowly over years; for a device that sleeps for months, this can dominate.
- Discharge rate — high currents lower the delivered capacity (Peukert's effect), pronounced in lead-acid and alkaline.
Frequently asked questions
- mAh or Wh — which should I use?
- mAh is simplest when the load is a current and the voltage is roughly constant. Wh (watt-hours) is better across different voltages — enter the battery voltage and the tool converts. Energy in Wh = capacity in Ah × voltage.
- What derating should I pick?
- 80% is a sane starting point for room-temperature use above a modest cutoff. Drop toward 60–70% for cold environments, high discharge rates, or long storage; raise it toward 90% only for gentle, warm, freshly-charged use.
- Does this work for a power bank or solar setup?
- Yes for the battery side — use Wh and your device's average power. Solar recharge is a separate balance (harvest vs consumption) this tool doesn't model.
- How do I lower average current?
- Shorten the active burst, deepen sleep (lower sleep current), or transmit less often. The duty-cycle mode lets you try each and watch the estimate move.